This invention relates to an enhanced oil recovery method for improving hydrocarbon recovery. More particularly, the invention is a method of improving carbon dioxide flooding by adding trichloroethane to carbon dioxide for floods.
The injection of carbon dioxide into underground reservoirs has become one of the chief methods of enhanced oil recovery practiced in the field. A wide variety of carbon dioxide injection schemes have been tried ranging from flooding under miscible to conditionally miscible to immiscible conditions. Injection schemes have also been applied in a cyclic injection and production fashion on single wells as well as in a flood front driving carbon dioxide through a reservoir from injection to production wells.
Although laboratory miscible slimtube floods have generally displaced over 90% of crude oil in slimtubes, and immiscible laboratory core floods have achieved recoveries nearly as great, recovery efficiencies in the field have been much lower. As in other methods of sweeping reservoirs such as water flooding, steam flooding and surfactant flooding, sweep efficiencies in the field are relatively low due to a lack of reservoir homogeneity and viscosity differences between the flooding medium and the underground hydrocarbons.
These problems have been decreased for aqueous floods by the addition of water soluble polymers such as polysaccharides and polyacrylamides to increase flood viscosity. Similar approaches have been tried with hydrocarbon and carbon dioxide floods with generally poor results.
The New Mexico Petroleum Recovery Research Center has performed tests with the use of high molecular weight polymers for increasing carbon dioxide viscosity. Extensive testing on a number of commercially available polymers has failed to find a solution. High molecular weight polymers do not have a sufficient solubility to alter carbon dioxide viscosity. These tests have been reported in Heller, J. P., Dandge, D. K., Card, R. G., and Donaruma, L. G., "Direct Thickeners for Mobility Control of CO.sub.2 Floods," SPE Journal, Oct. 1985.
Researchers at the New Mexico Petroleum Recovery Research Center have, however, found a class of compounds which gives substantial viscosity increases to alkane solvents by forming associations with large numbers of molecules. The organo-tin fluorides, such as tributyl-tin fluoride and diamylbutyl-tin fluoride can substantially increase the viscosity of hydrocarbon solvents. Please see, U.S. Pat. No. 4,607,696 and Dunn, P. and Oldfield, D., "Tri-n-Butyl Tin Fluoride: Novel Coordination Polymer in Solution," Journal of Macromolecular Science, Vol. A4(5) (1970) pg. 1160-76.
Two publications have noted relatively large increases in carbon dioxide densities with the addition of relatively low molecular weight compounds. These publications did not mention viscosity. See Paulaitis, M. E., Penninger, J. M. L., Gray, Jr., R. D., and Davidson, P., Chemical Engineering at Supercritical Fluid Conditions, Ann Arbor Science (1983) pg. 31-80; and Snedaker, R. A., Ph.D. Thesis entitled "Phase Equilibrium In 10 Systems with Supercritical Carbon Dioxide," Princeton University (1957).
The ability to predict the viscosity of a carbon dioxide and decane mixture by two correlations between density and viscosity was compared with actual measurements in Cullick, A. S. and Mathis, M. L., Journal of Chemical Engineering Data, Vol. 29 (1984) pg. 393-6.
U.S. Pat. No. 4,800,957 discloses a method of recovering hydrocarbons by injecting a mixture of carbon dioxide and a polar alcohol or polar glycol additive having less than about 9 carbon atoms. The alcohol or glycol additive comprises about 0.1% to about 10% by weight of the injected mixture. Significant increases in viscosity were noted.
A study of 1,1,1-Trichloroethane and carbon dioxide mixtures was made to model vapor-liquid equilibria and check the accuracy of several predictive equations. This is reported in Fink, Samuel, D., and Hershey, Harry C., "Modeling the Vapor-Liquid Equilibria of 1,1,1-Trichloroethane+Carbon Dioxide and Toluene+Carbon Dioxide at 308, 323, and 353 K," Ind. Eng. Chem. Res., Vol. 29 (1990), pp. 295-306. The reference does not discuss the behavior of a trichloroethane/carbon dioxide mixture at or above carbon dioxide critical temperatures or any applicability of such a system to hydrocarbon flooding.